(1) Field of the Invention
The present invention relates to curved honeycomb structural bodies formed by extruding a molding material such as a ceramic or a resin.
(2) Related Art Statement
In general, honeycomb structural bodies having a number of through holes are molded by extrusion. It is not easy for molding engineers to produce such honeycomb structural bodies by a process other than the extruding process. Therefore, in the conventional honeycomb structural bodies which are molded by extruding a material such as ceramic or resin, straight passages (through holes) are usually formed in an extruding direction.
Although a concept (configuration) of a honeycomb structural body having a curved shape can be illustrated and represented, such a curved honeycomb structural body cannot be easily actually produced. This is clear in view of the fact that it is difficult to finely define and constitute a number of curved passages with walls having a uniform thickness.
Japanese Patent application Laid-open No. 52-78,965 discloses a process for producing curved honeycomb structural bodies, in which lengths of passages of a die itself positioned in an extruding section of an extruder are variable to curve the honeycomb structural body toward a side of straight long passages having larger flow resistance.
However, according to this curved honeycomb structural body-producing process, since the widths of through holes (passages) of the extruding die are constant, the thicknesses of the interior walls extending in a curved direction are constant in the case of the honeycomb structural body extruded through the extruding die.
Therefore, in the curved honeycomb structural bodies obtained by the above process, the axial lengths of the interior walls are greater on the radially outer side of the curved portion than on the radially inner side thereof from the structural standpoint of view. Consequently, the thermal stress on the radially outer portion of the curved portion due to application and removal of thermal load is greater than that that on the radially inner side of the curved portion. Therefore, the interior walls on the radially outer side of the curved portion are likely to be fractured or cracked.
For example, when the above curved honeycomb structural body is used as a catalyst carrier for an automobile exhaust gas purification catalyst converter, the honeycomb structural body is placed in a metallic casing through an elastic body so that the catalyst carrier will not be fractured or cracked, in view of the fact that the honeycomb structural body is made of a brittle ceramic material such as cordierite. Such a catalyst converter ordinarily reaches temperatures around 800.degree. C. In general, when the catalyst converter is heated from room temperature to an operating temperature (high temperature), the thermal expansion of the metallic casing holding the honeycomb structural body is greater than that of the honeycomb structural body, the radially outer side of the metallic casing becomes greater. Consequently, as shown in FIG. 8, the honeycomb structural body 5 placed in the metallic casing 8 is subjected to a thermal load which makes the curved degree of the honeycomb structural body greater, with the result that the thermal expansion of the honeycomb structural body becomes greater on the radially outer side to make the honeycomb structural body easily fracturable.